41 C.F.R. § 301-13.1
Conditions of payment for additional travel expenses for special needs
In accordance with the Rehabilitation Act of 1973, as amended (29 U.S.C. 701 et seq.) and 5 U.S.C. 3102, an agency will pay additional travel expenses when necessary to reasonably accommodate a special physical need that is clearly visible and discernible; or substantiated in writing by a competent medical authority. Agencies should authorize and administer the payment to reasonably accommodate employee(s) with special needs.